In paleoclimatology and paleothermometry there is interest in determining the proportions of the oxygen isotopes 16O and 18O in solid samples.
From the work “Stable Isotope Geochemistry” of J. Hoefs, 4th Completely Revised, Updated and Enlarged edition, Springer, Berlin, Heidelberg, New York, Barcelona, Budapest, Hong Kong 1997, various methods of liberating oxygen from samples are known. Thus silicate-based samples of solids to be investigated are evaporated according to the state of the art, whereby oxygen is liberated and the analysis can proceed therewith. The oxygen can be investigated directly by mass spectroscopy as to its isotopes. Alternatively, the oxygen can be transformed in a further reaction with carbon into CO or CO2. The determination of the isotope ratio is then effected by mass spectroscopic investigation of the CO or CO2 resulting from reaction with graphite.
Another method which has come into use mainly with silicates and oxides is based upon the fluorination of the substances whose oxygen isotopes are to be analyzed. In this case, the oxygen is liberated by means of F2 or BrF5 in nickel cylinders at 500 to 600° C. The oxygen thereafter is generally transformed to CO2 on hot graphite and is then subjected to the mass spectroscopic measurement. Frequently the sample to be investigated requires a prepurification, because foreign molecules or groups which are detrimental to the analysis can be incorporated in the sample. Thus, for example biogenic silicates like the shells of diatoms frequently contain water or OH groups which make an analytic oxygen isotope determination very difficult. Should an analysis of the isotope composition of the oxygen nevertheless have to be carried out, the samples frequently initially must be freed from the foreign substances or molecules by preliminary purification steps which can falsify the analysis result. A method of liberating oxygen from the sample to be tested is the laser vaporization of partial regions of solids. In this method it is not possible to avoid a slight fractionation, which means a shift in the oxygen isotope ratio of the solid sample to be analyzed and which is based upon the somewhat different conditions in the boundary regions of the evaporation process. The heating of the solid sample by laser has therefore significant drawbacks with respect to the representativity or reproducibility of the results with large samples which can be heated only at point-like regions of the entire solid sample by the laser beam and thus makes the isotope analysis accessible. In the case of inhomogeneities in a sample, the results of the respective measurements of the ratios of the oxygen isotopes are not truly representative for the entire sample. To obtain representative results, therefore, many measurements are required whose results must be averaged.